This invention relates to an electrosurgical instrument for the treatment of tissue in the presence of an electrically conductive fluid medium, to electrosurgical apparatus including such an instrument, and to an electrode unit for use in such an instrument.
Endoscopic electrosurgery is useful for treating tissue in cavities of the body, and is normally performed in the presence of a distension medium. When the distension medium is a liquid, the technique is commonly referred to as underwater electrosurgery, this turn denoting electrosurgery in which living tissue is treated using an electrosurgical instrument with a treatment electrode or electrodes immersed in liquid at the operation site. A gaseous medium is commonly employed when endoscopic surgery is performed in a distensible body cavity of larger potential volume in which a liquid medium would be unsuitable, as is often the case in laparoscopic or gastroenterological surgery.
Underwater surgery is commonly performed using endoscopic techniques, in which the endoscope itself may provide a conduit (commonly referred to as a working channel) for the passage of an electrode. Alternatively, the endoscope may be specifically adapted (as in a resectoscope) to include means for mounting an electrode, or the electrode may be introduced into a body cavity via a separate access means at an angle with respect to the endoscope--a technique commonly referred to as triangulation. These variations in technique can be subdivided by surgical speciality, where one or other of the techniques has particular advantages given the access route to the specific body cavity. Endoscopes with integral working channels, or those characterised as resectoscopes, are generally employed when the body cavity may be accessed through a natural opening--such as the cervical canal to access the endometrial cavity of the uterus, or the urethra to access the prostate gland and the bladder. Endoscopes specifically designed for use in the endometrial cavity are referred to as hysteroscopes, and those designed for use in the urinary tract include cystoscopes, urethroscopes and resectoscopes. The procedures of transurethal resection or vaporisation of the prostate gland are known as TURP and EVAP respectively. When there is no natural body opening through which an endoscope may be passed, the technique of triangulation is commonly employed. Triangulation is commonly used during underwater endoscopic surgery on joint cavities such as the knee and the shoulder. The endoscope used in these procedures is commonly referred to as an arthroscope.
Electrosurgery is usually carried out using either a monopolar instrument or a bipolar instrument. With monopolar electrosurgery, an active electrode is used in the operating region, and a conductive return plate is secured to the patient's skin in a position remote from the operating site. With this arrangement, current passes from the active electrode through the patient's tissues to the external return plate. Since the patient represents a significant portion of the circuit, input power levels have to be high (typically 150 to 250 watts), to compensate for the resistive current limiting of the patient's tissues and, in the casse of underwater electrosurgery, power losses due to the fluid medium which is rendered partially conductive by the presence of blood or other body fluids. Using high power with a monopolar arrangement is also hazardous, due to the tissue heating that occurs at the return plate, which can cause severe skin burns. There is also the risk of capacitive coupling between the instrument and patient tissues at the entry point into the body cavity.
When performing surgery in body cavities, vital structures often lie in close proximity to the site of application, and these structures may be damaged by the collateral spread of the electrosurgical effect. Also of concern when using monopolar electrosurgery is that the operating voltage is elevated to overcome the resistive current limiting of the patient's tissues or to overcome carbonisation of the application electrode. Arcing by direct coupling to adjacent structures, or through breaches in insulation, may produce accidental tissue damage outside the narrow field of view of the endoscope. There is also the risk of capacitive coupling between the instrument and the patient's tissues at the entry point into the body cavity such that electrosurgical energy may be coupled to tissue at the entry point. This coupled energy can sometimes be sufficient to cause burning These risks of using monopolar electrosurgery during endoscopic procedures are now well recognised, and have driven a move towards adoption of bipolar surgery.
With bipolar electrosurgery, a pair of electrodes (an active electrode and a return electrode) are used together at the tissue application site. This arrangement has advantages from the safety standpoint, due to the relative proximity of the two electrodes so that radio frequency currents are limited to the region between the electrodes. However, the depth of effect is directly related to the distance between the two electrodes; and, in applications requiring very small electrodes, the inter-electrode spacing becomes very small, thereby limiting tissue effect and the output power. Spacing the electrodes further apart would often obscure vision of the application site, and would require a modification in surgical technique to ensure correct contact of both electrodes with the tissue.
There are a number of variations to the basic design of the bipolar probe. For example, U.S. Pat. No. 4,706,667 describes one of the fundamentals of the design, namely that the ratio of the contact areas of the return electrode and of the active electrode is greater than 7:1 and smaller than 20:1 for cutting purposes. This range relates only to cutting electrode configurations. When a bipolar instrument is used for desiccation or coagulation, the ratio of the contact areas of the two electrodes may be reduced to approximately 1:1 to avoid differential electrical stresses occurring at the contact between the tissue and the electrodes.
The electrical junction between the return electrode and the tissue can be supported by wetting of the tissue by a conductive solution such as normal saline. Both monopolar and bipolar probe arrangements often provide a means of suction and irrigation, primarily intended to wash the operative site. In such a case, the active electrode is retracted within the irrigation sheath to enable direct contact of the sheath with the tissue without the risk of mechanical damage to the tissue by the exposed electrode. No surgical effect can be produced with the electrode retracted, or during the passage of saline. As a secondary benefit, this arrangement allows the wetting of tissue to reduce contact impedance.
In bipolar needle arrangements, one of the obvious limitations is that the active electrode must be completely buried in the tissue to enable the return electrode to complete the circuit. Another problem is one of orientation: even a relatively small change in application angle from the ideal perpendicular contact with respect to the tissue surface, will change the electrode contact area ratio, so that a surgical effect can occur in the tissue contacting the return electrode.
The applicants have developed a bipolar instrument suitable for underwater electrosurgery using a conductive liquid or gaseous medium. This electrosurgical instrument for the treatment of tissue in the presence of a fluid medium, comprises an instrument body having a handpiece and an instrument shaft and an electrode assembly, at one end of the shaft. The electrode assembly comprises a tissue treatment electrode which is exposed at the extreme distal end of the instrument, and a return electrode which is electrically insulated from the tissue treatment electrode and has a fluid contact surface spaced proximally from the exposed part of the tissue treatment electrode. In use of the instrument, the tissue treatment electrode is applied to the tissue to be treated whilst the return electrode, being spaced proximally from the exposed part of the tissue treatment electrode, is normally spaced from the tissue and serves to complete an electrosurgical current loop from the tissue treatment electrode through the tissue and the fluid medium. This electrosurgical instrument is described in the specification of the applicants' co-pending British Patent Application No. 9512889.8.
The electrode structure of this instrument, in combination with a conductive distension medium largely avoids the problems experienced with monopolar or bipolar electrosurgery. In particular, input power levels are much lower than those generally necessary with a monopolar arrangement (typically 100 watts). Moreover, because of the relatively large spacing between its electrodes, an improved depth of effect is obtained compared with conventional bipolar arrangements.
This type of electrosurgical instrument is designed primarily for use in a saline environment, and so cannot be used in open air or gas-filled operating environments.
The aim of the invention is to provide an irrigated bipolar electrosurgical instrument that can be used in open air or gas-filled environments, in body fluids, or by insertion into tissue by the creation of a conductive fluid environment around the tip of the instrument.